The mammalian olfactory bulb plays a key role in discrimination of thousands of odor molecules but the underlying mechanisms remain to be resolved. It's local circuits are mainly built up with reciprocal synapses formed between dendrites of principle neurons and inhibitory interneurons. This unique organization implies an important role for olfactory bulb dendrites in the central processing of odor information, but little is known about the functional properties of these dendrites and their dendrodendritic synapses, mostly due to their inaccessibility and small size. The aim of this project is to use recently developed techniques for dual patch clamp recordings combined with infrared videomicroscopy to record directly from mitral cell dendrites in rat olfactory bulb slices. As principle neurons in the olfactory bulb, the mitral cells have one apical primary dendrite and several basal secondary dendrites. The primary dendrite connects to a single glomerulus, receiving olfactory nerve input and forming dendrodendritic synapses at its distal tuft, whereas the secondary dendrites extend horizontally over a long distance and make numerous dendrodendritic reciprocal synapses with dendritic spines of inhibitory granule cells. Three major hypotheses will be tested: 1) action potentials can be initiated in the distal glomerular tuft of the primary dendrite, and this distal dendritic initiation has specific functions, i.e. providing a mechanism enabling local synaptic output in the distal primary dendrite independent of the output through the axon; 2) active mechanisms in the primary dendrite are involved in the transmission of distal olfactory nerve input to the soma and axon initial segment; and 3) action potentials can also propagate actively in the mitral cell secondary dendrites, and such propagation is essential for activating distal dendrodendritic reciprocal synapses. These experiments will be aimed at the long-term objective of obtaining critical information on the functional principles of olfactory bulb dendrites and how these dendrites contribute to odor information processing. The progress toward this objective will yield novel insights into how humans can discriminate thousands of different odors, and into the neural basis of disorders of smell.